Dating back to the 1880s, communication and telecommunication close-coupled ear “receiver” technology was largely limited to “narrowband” devices delivering sound to the ear between approximately 300-3300 Hz. Over the last twenty years, the industry has introduced “wideband” instruments capable of achieving a 150-7000 Hz bandwidth range. While the wideband technology was able to achieve an increase in bandwidth over the earlier narrowband technology, the range still falls short of the 20-20,000 Hz range that can be achieved by human speech and hearing.
This wideband technology has now matured in performance and availability to achieve significant consumer acceptance. Nevertheless, recent major advances in digital electronics and voice communication facilities bandwidth have given rise to a desire to mimic the “hi-fi” industry (i.e. to utilize the full spectral capability of human hearing) not only for voice, but for music as well. Thus, it is desirable to provide audio receiver technology that goes beyond even 150 Hz-7 kHz wideband range.
Conventional wideband supra-aural and supra-concha earphones, headsets, and handsets use a single receiver transducer to cover the 150-7000 Hz bandwidth range. It is difficult to achieve response bandwidths outside of this wideband frequency range due to, among other reasons, (i) ear leakage, (ii) constraints on the product's interior housing air volume, and (iii) the relatively high mass and thus low resonance frequency of the receiver diaphragm, which results in a limited upper frequency response range.
Historically, hi-fidelity speaker systems and commercially available in-canal earphones use multiple transducers to deliver sound over different portions of the frequency spectrum. Such speaker systems work into an entirely different complex acoustical load impedance than the ear load of supra-concha and supra-aural earphones, and such speaker systems are not constrained in size. Similarly, small intra-concha (ear-bud) and insert (canal) earphone products exist that use dual-drive transducers. These very small earphones drive directly into the relatively low volume and thus acoustically-stiff ear canal, as opposed to the lower stiffness and far larger air volume of the human outer ear seen by supra-aural and supra-concha earphones. In addition, the outer ear also allows sound to leak to the ambient. Therefore, earphones that are designed to be worn in a supra-concha or supra-aural fashion require far greater electro-acoustic efficiency and physical size than that required by their intra-concha and insert counterparts. Accordingly, it would be desirable to provide a supra-concha or supra-aural audio receiver technology that is capable of going beyond the conventional 150-7000 Hz wideband range in spite of the difficulties imposed by the low acoustical impedance of the large air volume of the outer ear and the associated sound leakage to the ambient.